You have a crumbled concrete rat slab. You have to break it up and remove loose particles of concrete, dig down a few extra inches and remove the dirt so you have the extra depth for a solid concrete base... Add concrete bonding agent to the mix, fill, and level concrete mix with the adjoining slab.

As said, that is a non-structural 2" thick slab just for housekeeping (to have a non-dirt floor). Round here we call them "mudslabs." Just buy basic concrete mix bags and use a trowel to float finish the top smooth and flush to the adjacent areas. It ain't meant to be pretty and you don't have to overthink it, though you can remove the pieces that looked cracked off already from the sledgehammering by the plumber.